Telomeres are essential for the maintenance of genomic integrity. Dyskeratosis congenita (DC) is a cancer- prone syndrome characterized by short telomeres. Affected patients have an increased risk for developing hematologic malignancies, specifically myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML). While DC is an inherited Mendelian disorder, germline mutations in telomerase and telomere components are identifiable in only two-thirds of families, leaving the causal mutations in the remaining cases uncharacterized. Mutations in DC genes also underlie inheritance in a subset of MDS and AML families. This fact, along with the observation that MDS-AML patients have short telomeres, has suggested an intimate role for telomere length in the genetics of these disorders. This project examines two aspects of DC genetics and biology of relevance to understanding MDS-AML pathogenesis. We aim to identify novel genes that are critical for telomere maintenance by studying genetically uncharacterized DC families in a registry we have established. Given the known limitations of traditional linkage approaches in small kindreds, the cohort we have compiled provides an ideal setting to apply next-generation sequencing technologies for the purpose of gene discovery. In Aim 2, we examine the biology by which short telomeres promote MDS-AML in an animal model of DC we have characterized. This murine model uniquely recapitulates human telomere length dynamics. The proposed studies in DC have particular significance for understanding the biology of MDS-AML since the telomere defect found in DC patients is universally acquired with aging, and the biology that underlies the increasing incidence of MDS-AML with age is not understood. Broadly, they have implications for understanding fundamental questions regarding the role of telomere length in cancer risk and progression.